The present invention relates to assembly methods of a motor housing and a stator core of a sealed type motor-driven compressor.
In a sealed type motor-driven compressor, a sealed housing accommodates, for example, a scroll type compressor mechanism and an electrical motor for driving the mechanism. The motor includes a rotational shaft, a rotor, and a stator. The rotational shaft of the motor is rotationally supported at a middle portion of a motor housing, which forms part of the sealed housing. The rotor is securely fitted to the outer circumferential surface of the rotational shaft. The stator is securely fitted to the inner circumferential surface of the motor housing through shrink fitting. The stator includes a cylindrical stator core and coils arranged along the inner circumference of the stator core. A technique for shrink-fitting the stator to the motor housing is disclosed in, for example, Japanese Laid-Open Patent Publication Nos. 2000-224787 and 2003-269335.
When the motor housing is formed of aluminum, which has smaller modulus of elasticity than iron, a pressure rise in the motor housing causes a relatively great increase of the inner diameter of the motor housing, as compared to the case in which the motor housing is formed of iron. Chlorofluorocarbon or carbon dioxide is used as refrigerant gas charged in a refrigerating circuit of a vehicle air conditioner. The maximum charging pressure of chlorofluorocarbon gas is approximately 1 to 2 MPa, while that of carbon dioxide is equal to or greater than 10 MPa. As long as the pressure in the motor housing is maximally 1 to 2 MPa, the stator is prevented from loosening with respect to the motor housing by a fastening interference defined in shrink fitting of the stator core and the motor housing. However, if carbon dioxide is used as the refrigerant, the pressure equal to or greater than 10 MPa is applied to the motor housing.
Accordingly, if carbon dioxide is used as the refrigerant and the motor housing formed of aluminum is employed, the motor housing must have a relatively large wall thickness for preventing the increase of the inner diameter of the motor housing. However, this increases the dimensions and weight of the compressor. Alternatively, the shrink fitting of the stator core and the motor housing may be performed with a sufficiently large fastening interference such that an effective fastening interference is maintained even if the inner circumference of the motor housing is increased. However, to provide a sufficiently large fastening interference, the shrink fitting must be performed at a relatively high temperature, leading to lowering of the strength of the motor housing, which is formed of aluminum. It is thus extremely difficult to increase the fastening interference for the shrink fitting.
In contrast, if the motor housing is formed of iron, the increase of the inner diameter of the motor housing, which is caused by the high pressure applied by the refrigerant gas, is extremely small. However, the motor housing formed of iron increases the weight of the compressor.